The invention relates to a process for reducing the total sulfur content of Claus off-gases.
The process for the preparation of elemental sulfur from hydrogen sulfide by partial oxidation thereof by means of oxygen or an oxygen-containing gas such as air, followed by reaction of the sulfur dioxide formed from the hydrogen sulfide with the remaining part of the hydrogen sulfide in the presence of a catalyst is known as the Claus process. This process, which is frequently used both at refineries and for working-up hydrogen sulfide recovered from natural gas, is carried out in a Claus plant comprising a combustion chamber followed by one or more catalyst beds, one or more condensers being arranged in between in which the reaction products are cooled and the separated liquid elemental sulfur is recovered. The various process steps can be represented by the following reaction equations: EQU 2H.sub.2 S + 30.sub.2 .fwdarw. 2H.sub.2 O + 2SO.sub.2 (1) EQU 4h.sub.2 s + 2so.sub.2 .revreaction. 4h.sub.2 o + 6/.sub.x S.sub.x (2)
while the total reaction is represented by equation (3): EQU 6H.sub.2 S + 3O.sub.2 .revreaction. 6H.sub.2 O + 6/.sub.x S.sub.x (3)
For temperatures below 500.degree. C the symbol x in the above equation has a value of 8.
Since the yield of recovered elemental sulfur relative to the hydrogen sulfide introduced is not completely quantitative, a certain amount of unreacted hydrogen sulfide and sulfur dioxide remains in the Claus off-gases. These gases are normally incinerated in a furnace--which converts the hydrogen sulfide to sulfur dioxide--which is subsequently discharged to the atmosphere through a high stack. The amount of elemental sulfur recovered depends to some extent on the total number of catalyst beds used in the Claus process. In principle, 98% of sulfur can be recovered when three beds are used.
Viewed in the light of the increasingly stringent requirements in respect of air pollution abatement, incineration of Claus off-gases is no longer a satisfactory means of disposal. Moreover, it involves a certain loss in sulfur yield.